Ceramic honeycomb structures composed of a multitude of cells or passages separated by thin walls running parallel to the longitudinal axis of the structure with, in some instances, discontinuities designed to extend transversely through those walls are well known to the art. Such articles have been employed extensively as filters for fluids and as heat exchangers. More recently, the walls of those structures have been coated with a catalyst capable of converting noxious fumes from the discharge gases of internal combustion engines and wood stoves into non-noxious components. As can readily be appreciated, the environment inherent in those recent applications demands that the structures exhibit a complex matrix of chemical and physical properties. For example, the mechanical strength of the structure must be sufficient to withstand the mechanical forces encountered in mounting the structure plus the physical vibrations and pressures of the emission gases experienced in use along with high refractoriness, high thermal shock resistance, low thermal expansion, and good resistance to physical abrasion from particles in the emission gases and to chemical attack from the fumes therein.
Numerous materials have been proposed and tested as substrates for catalyst-coated honeycomb structures including alumina-silica, alumina, zirconia-alumina, zirconia-magnesia, mullite, zircon, zircon-mullite, titania, spinel, zirconia, Si.sub.3 N.sub.4, and carbon. Only two materials, however, have actually seen any substantial service in that utility; viz., cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) and beta-spodumene solid solution (Li.sub.2 O.Al.sub.2 O.sub.3.2-8SiO.sub.2).
Beta-spodumene solid solution has a very low coefficient of thermal expansion but its use temperature (&lt;1200.degree. C.) is so low as to severely restrict its utility in this application. Cordierite or cordierite+ a compatible refractory phase, commonly mullite, has been employed extensively as substrate structures for automotive catalytic converters. Unfortunately, those substrates do not fully meet the combined demands of high thermal shock resistance and high service temperature at the same time. Substrates are occasionally subjected to short high temperature excursions, e.g., an automobile ignition malfunction in which the temperature exceeds 1465.degree. C., the melting point of cordierite. Moreover, cordierite cannot meet the high temperature requirements of such applications as automotive light-off catalysts, catalytic converters for truck engines, molten metal filters, and high temperature heat exchangers. To raise the service temperature, the cordierite has in some instances been diluted with a highly refractory phase such as mullite. In so doing, however, the coefficient of thermal expansion is raised and the resistance to thermal shock is substantially decreased. Moreover, the service temperature is raised only for short time transient exposures because the cordierite fraction of the body will still melt at 1465.degree. C. Accordingly, materials displaying higher refractoriness and thermal shock resistance have been sought for that application. U.S. Pat. Nos. 4,118,240 and 4,327,188 are illustrative of such work.
The former patent notes that, upon firing, microcracks develop in the body such that the average coefficient of thermal expansion thereof is quite low. Unfortunately, however, the presence of those microcracks sharply reduces the mechanical strength of the body. The mechanism underlying the microcracking phenomenon is explained in the patent in this manner. The Al.sup.+3 sites in the aluminum titanate crystals are significantly larger than the ionic radius of Al.sup.+3, so that Al.sup.+3 ions are moved out of the crystals sites when the crystals are subjected to high temperatures. This results in a gradually increasing amount of Al.sub.2 O.sub.3 being formed and the coefficient of thermal expansion of the product gradually increasing. Ti.sup.+3 ions are formed via the reduction of Ti.sup.+4 ions and the former move into the vacancies left by the Al.sup.+3 ions. Hence, where aluminum titanate is exposed to high temperatures in a reducing environment, the decomposition of aluminum titanate through the change in crystal lattices can occur relatively rapidly.
The patent observed that the prior art had proposed the inclusion of Mg.sup.+2, Fe.sup.+3, or Cr.sup.+3 ions to substitute for part of the Al.sup.+3 ions. However, because the ionic radius of those three ions was only slightly larger than that of Al.sup.+3 ions, the desired inhibiting effect upon the decomposition of aluminum titanate crystals was small. The patent disclosed that inhibition of the decomposition of aluminum titanate crystals could be significantly enhanced through the substitution of Sn.sup.+4 and/or rare earth element ions for a portion of the Al.sup.+3 ions. Lanthanum, cerium, and yttrium were explicitly reported as suitable rare earth elements for the inventive practice. SiO.sub.2 was also incorporated to improve the mechanical strength of the bodies.
U.S. Pat. No. 4,327,188 is directed specifically to the production of ceramic honeycombs to be utilized as catalyst substrates. The articles were prepared from a combination of aluminum titanate and SiO.sub.2 to which Y.sub.2 O.sub.3 and/or La.sub.2 O.sub.3 and/or CeO.sub.2 may optionally and desirably be included. SiO.sub.2 functions as a sintering aid and the rare earth elements not only perform as sintering aids, but also inhibit decomposition of aluminum titanate crystals when exposed to high temperatures. The amount of Y.sub.2 O.sub.3 and/or La.sub.2 O.sub.3 and/or CeO.sub.2 required to be included can be reduced through adding a minor amount of Fe.sub.2 O.sub.3. The honeycombs were asserted to be operable for continuous use at temperatures higher than 1450.degree. C. and for short exposures to temperatures up to 1650.degree. C.
Nevertheless, because of the severe environment to which the catalyst-coated honeycomb structure is subjected in emission control and other applications, the modified aluminum titanate bodies described above have not been fully satisfactory. Hence, where fabricated honeycombs are to be used as carriers for a catalyst, the ceramic must exhibit four critical characteristics; viz., very high refractoriness, high porosity for carrying the catalyst wash coat combination, high mechanical strength to permit the use of very thin walls in the honeycomb, thereby more effectively using the catalyst, and high thermal shock resistance. The intrinsic mechanical strength of the ceramic is of special criticality inasmuch as higher porosity results in lower strength. Consequently, a compromise must be struck between the desired high porosity and the needed mechanical strength.
Sintered bodies consisting essentially of aluminum titanate and mullite have been known to the art. Because the melting point of mullite is about 1880.degree. C. and that of aluminum titanate is about 1860.degree. C., the body resulting from firing a mixture of those two components would be expected to be highly refractory. Mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) consists in weight percent of about 71.8% Al.sub.2 O.sub.3 and 28.2% SiO.sub.2. Aluminum titanate (Al.sub.2 O.sub.3.TiO.sub.2) consists in weight percent of about 56.06% Al.sub.2 O.sub.3 and 43.94% TiO.sub.2.
The appended drawing comprises a ternary composition diagram of the Al.sub.2 O.sub.3 --TiO.sub.2 --SiO.sub.2 system in terms of weight percent. Point A designates the Al.sub.2 O.sub.3.TiO.sub.2 composition and Point B the mullite composition.
In WADC (Wright Air Development Center) Technical Report 53-165, June, 1953, Aluminum Titanate and Related Compounds, N. R. Thielke fired and tested a series of bodies having compositions along the join between Al.sub.2 O.sub.3.TiO.sub.2 and mullite, and also along the line connecting Points A and C.
British Pat. No. 1,081,142 describes the firing of compositions within the Al.sub.2 O.sub.3 --TiO.sub.2 --SiO.sub.2 ternary to form bodies exhibiting melting points from 1600.degree. C. to greater than 1800.degree. C. and coefficients of thermal expansion ranging from -15 to 15.times.10.sup.-7 /.degree.C. Sintering was carried out at 1400.degree.-1600.degree. C. Li.sub.2 O, ZnO, and the alkaline earth metal oxides were noted as useful sintering aids. The patent indicates that the addition of such highly refractory materials as ThO.sub.2, ZrO.sub.2, Y.sub.2 O.sub.3, CeO.sub.2, carbides, nitrides, borides, and sulfides raises the temperature at which the body can be used. A like phenomenon is stated to occur when a portion of the SiO.sub.2 is replaced with B.sub.2 O.sub.3 and/or P.sub.2 O.sub.5.
Whereas no identification of the crystal phases present in the sintered products is provided, some of the compositions encompassed within the specification would yield Al.sub.2 O.sub.3.TiO.sub.2 and mullite crystals. Thus, the compositions are broadly stated to consist of Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 in the mole ratio of Al.sub.2 O.sub.3 :0.05-1.5 SiO.sub.2 :0.5-1.5 TiO.sub.2.
French Patent No. 1,349,020 discloses sintered refractory bodies consisting essentially, in weight percent, of 25-70% Al.sub.2 O.sub.3, 15-75% TiO.sub.2, 0-20% MgO, and 0-40% SiO.sub.2 which are asserted may have melting temperatures of 1700.degree.-1850.degree. C. and coefficients of expansion of zero or less.
No identification of the crystal phases present in the final product was supplied, but the SiO.sub.2 -containing bodies could very well have a combination of Al.sub.2 O.sub.3.TiO.sub.2 and mullite crystals. The area bounded within Points D, E, F, G, H, D of the drawing reflects the Al.sub.2 O.sub.3 --TiO.sub.2 --SiO.sub.2 compositions (exclusive of MgO) disclosed in the patent.